Understanding the stars is all about models. Astronomers build mathematical models to explain how stars generate energy, how they evolve, and how they die. The models make predictions about a star’s brightness, its temperature, its size, and other parameters. So to verify how well the models work, astronomers compare their predictions to measurements of real stars.

Consider the star Epsilon Geminorum. It’s in the constellation Gemini, which is in the east in early evening. Epsilon Geminorum is well to the right of Gemini’s brightest stars, the “twins” Castor and Pollux.

A technique called photometry reveals a star’s temperature by measuring its color. Epsilon Geminorum is yellow-orange, so it’s fairly cool.

And astronomers get a star’s true brightness by comparing its distance to how bright it looks in the night sky. Epsilon is about 900 light-years away, so it’s thousands of times brighter than the Sun.

The Moon periodically passes in front of Epsilon, blocking it from view. Measuring how long it takes to disappear behind the Moon reveals the star’s size. Astronomers also measure the size by combining the view from several telescopes, which produces ultra-sharp images. These techniques reveal that Epsilon Geminorum is about 150 times the Sun’s diameter, making it a supergiant.

All of these numbers match the models quite well — showing that astronomers have a pretty good grasp on how the stars work.